Abstract: A combustion chamber capable of being directly fueled with green bark, green sawdust and the like material formed of refractory material, the chamber having a grate therein at the feed end thereof disposed towards the bottom of the chamber and a reduced outlet and having means for delivering particles of fuel material generally downwardly towards the grate while separating and enveloping the particles with pressurized turbulent air, and means for directing air upwardly through the grate against the downwardly directed air enveloped fuel particles in sufficient volume and with sufficient pressure and velocity to effect turbulent suspension of a substantial proportion of the fuel particles above the grate, the chamber further being air tight apart from the fuel and air input feeds and the outlet whereby the chamber is adapted to operate on a positive pressure with respect to the external ambient pressure.